1. Field of the Invention
The present invention relates to a membrane electrode assembly for a solid polymer electrolyte fuel cell, and in particular, relates to a membrane electrode assembly which provides high output with a graphitized catalyst.
2. Description of Related Art
Fuel cells use hydrogen gas as a fuel gas and generate energy by an electrochemical reaction of hydrogen and oxygen. There are various types of fuel cells, including a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid electrolyte type fuel cell, and a solid polymer electrolyte fuel cell. Among these, a solid polymer electrolyte fuel cell can start operation at a normal temperature and can also provide higher output with a small structure, and it thus holds great promise as a power source for two-wheeled or four-wheeled vehicles, or as a portable power supply. In the case in which such a solid polymer electrolyte fuel cell is actually used, an electrode structure, which is a basic unit of a fuel cell, is held between separators to form a single cell, and then several tens to hundreds of single cells are combined from a stack (battery).
A single cell generally includes an anode electrode and a cathode electrode that are provided with a diffusion layer and a catalyst layer, a membrane electrode assembly (MEA) that is formed from a polymer electrolyte membrane sandwiched by these electrodes, and a separator that sandwiches this membrane electrode assembly.
The catalyst layer contains catalyst supporting particles in which a catalyst metal is supported on a carrier. As a means for improving the durability of this catalyst layer, it is disclosed that the durability of the catalyst carrier is improved by previously applying high temperature heat processing to a carrier. In addition, carbon is usually used as a catalyst carrier; however, it is also disclosed that water repellency and the anti-corrosion characteristics of the catalyst layer are improved by applying heat treatment to the carbon to be graphitized. (For example, see Patent Documents 1 to 4)
Patent Document 1
Japanese Patent Application Laid-Open No. 2000-268828
Patent Document 2
Japanese Patent Application Laid-Open No. 2001-357857
Patent Document 3
Japanese Patent Application Laid-Open No. 2002-015745
Patent Document 4
Japanese Patent Application Laid-Open No. 2003-036859
According to Patent Documents 1 to 4, it is disclosed that durability is improved by using graphitized carbon as a catalyst layer. However, when carbon black whose specific surface area is 300 m2/g or more is heat-treated as a graphitized catalyst carrier, in particular, when it is heat-treated at 2,500° C. (degrees Celsius) or more, the specific surface area is greatly reduced. Therefore, when a precious metal catalyst supports platinum or a platinum alloy, sufficient power generation performance cannot be obtained.
In addition, when carbon black is graphitized by high temperature heat processing, a sintering of carbon particles occurs, and then a large and firm aggregate having a diameter of 10 μm (micrometers) or more is formed. When a membrane electrode assembly is formed, an aggregate like this alters a polymer electrolyte membrane and reduces the durability. Furthermore, the thickness of an electrode catalyst layer becomes uneven, and generating efficiency is reduced.
Conventionally, while there has been an advantage in that durability can be improved by applying heat treatment to carbon black, there has been a problem in that performance as a catalyst is adversely reduced because the reduction or the flocculation of the specific surface area occurs by heat treatment.